Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder system. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown or otherwise formed to the desired shape or patterns, and then cured with the use of catalyst and/or heat to a solid, cured state.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at a low temperature, to avoid the need for high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as "cold-box" processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the material is blown into a box of the desired shape. Curing of the binder is carried out by passing a gaseous catalyst at ambient temperatures through the molded resin-coated material. In such processes, the binder components normally comprise a polyhydroxy component and a polyisocyanate component. These cure to form a polyurethane in the presence of a gaseous amine catalyst.
Another group of binder systems which do not require gassing or heating in order to bring out curing are known as "no-bake" systems. These "no-bake" systems also frequently employ an aggregate material, such as sand coated with a polyhydroxy component and a polyisocyanate component. In this case, the coated sand is usually mixed with a liquid tertiary amine catalyst just before the sand is placed into a holding pattern or core box, and the material is allowed to cure at ambient temperatures or slightly higher.
Although developments in resinous binder systems which can be processed according to the "cold-box" or "no-bake" processes have resulted in the provisions of useful systems, workers have continually sought to improve the binders of these systems. One such improvement is disclosed in U.S. Pat. No. 4,546,124 issued on Oct. 8, 1985 to Laitar et al. This patent, which describes an alkoxy modified phenolic resole resin as the polyhydroxy component of the polyurethane binder, is incorporated herein by reference in its entirety.
Various other workers have disclosed techniques for modifying phenolic resins. However, none of these modified resins have been used as components of binders for foundry cores and molds. For example, U.S. Pat. No. 2,376,213 discloses that water miscible phenolic resins can be prepared by the reaction of phenol with an excess of formaldehyde in the presence of a polyhydroxy alcohol using an alkali metal hydroxide as a catalyst. On the other hand, U.S. Pat. No. 3,156,670 discloses the formation of a water insoluble liquid phenolic resin. The phenolic nuclei are said to be linked together by dihydric glycol residues through aliphatic ether linkages. The reaction between the phenol, formaldehyde and the glycol is carried out using an alkaline catalyst and then the reaction is completed by dehydration under acidic conditions.
Preparation of molding materials is disclosed in U.S. Pat. No. 3,894,981. These are prepared by the reaction of phenol, an aldehyde, and a monohydric or dihydric alcohol in the presence of a filling agent such as wood flour or asbestos. The reaction is carried out under mildly acidic anhydrous conditions at high temperatures. Another modified phenolic resin said to be useful for making resin impregnated filter paper is disclosed in U.S. Pat. No. 4,150,194. It is prepared by the reaction of phenol with an aqueous aldehyde and a glycol in the presence of an alkaline catalyst.
Three patents disclose modified phenolic resins which have been used to make urethane foams. In U.S. Pat. No. 4,404,334, phenol, anhydrous formaldehyde and a glycol are reacted in the presence of zinc acetate catalyst under nonrefluxing conditions. The products are said to be useful to plasticize conventional phenolic resins or to prepare improved heat and flame resistant polyurethane foams. U.S. Pat. Nos. 4,448,951 and 4,473,669 disclose the reaction of phenol and nonaqueous aldehydes with an alcohol or polyol in the presence of a divalent metal ion catalyst. The products were used to make solid urethane foams of low friability and low combustibility.
All of the resole resins previously used to make urethane foundry binders have possessed an undesirable formaldehyde odor. The release of appreciable quantities of formaldehyde into the atmosphere is undesirable from an environmental view point. Previous attempts to reduce the amount free formaldehyde in the resin has required long periods of heating and has produced resins of undesirably high viscosity.
We have now discovered a process whereby modified resole resins can be prepared which emit little formaldehyde and yet retain their desired range of viscosity. These resins can be used to make urethane binders particularly suitable for use in the "cold-box" and "no-bake" processes. Furthermore, when the resins are combined with polyisocyanates for use in the "cold-box" process, they give a mixture with better bench life than ones prepared with resole resins previously employed.